1. Field of the Invention
The present invention relates to a thermal transfer printer for transferring images to a recording sheet using a ink film coated with a heat-fusion type ink.
2. Description of the Related Arts
Conventional thermal transfer printers have a platen roller 1 and a retractable thermal head 2 or the like capable of making and breaking contact with said platen roller 1, as shown in FIG. 4. Recording sheet 3 is transported, by a transport means, between platen roller 1 and thermal head 2 so as to wrap around the outer surface of said platen roller 1.
An ink film 4, which is coated on one side with a heat-fusion ink, is fed so as to pass medially between the aforesaid recording sheet 3 and thermal head 3. The ink film 4 is fed from an ink film roll 5, guided by guide rollers 7 and 8, and rerolled onto ink film reel 6.
Because the ink film 4 used in the aforesaid conventional thermal transfer printer is an extremely thin film having a thickness of about 3 to 10 .mu.m to improve transfer efficiency, it is transported at an incline with unequal tension on the upstream and downstream side of the platen roller in the film transport direction when the ink film is fed, which at times produces a twist in the film and causes poor ink transfer to the recording sheet. Further, wrinkles are at times produced in the film during the ink film 4 transporting process. These wrinkles are thought to be caused by dislocation of ink film roll 5 and ink film reel 6 respectively used to feed the ink film to thermal head 2 and rewind the film thereafter, and by stretching the film 4 with different tension on the upstream and downstream side of the platen roller in the film transport direction due to the peeling process.
The latter case results in a great discrepancy in the quantity of ink transferred to recording sheet 3 in the width direction of film 4, because as ink film 4 is peeled from recording sheet 3 after the transfer, the peeling force causes a large disparity in the tension on ink film 4 on the upstream and downstream side of the platen roller in the film transport direction, such that when the ink transfer occurs after wrinkles are produced in the ink film, poor ink transfer will result.
In U.S. Pat. application Nos. 4,425,568 and 4,463,360, the inside of the ink film makes touching contact with a stress-absorbing roller disposed between the platen and the supply roller, the center of rotation of said stress-absorbing roller being inclined in a perpendicular direction relative to the surface of the ink film. By inclining the stress-absorbing roller, tension is equalized in the width direction of the ink film. However, the degree of parallelism between the film supply roller and the rerolling roller cannot be maintained, and the ink film cannot be transported with uniform tension.